Many diverse structures are made of layers that are not compatible when the layers are ground up and used in a regrind layer. For instance, barrier fuel tanks for automobiles typically include at least one layer comprising virgin HDPE, a barrier layer comprising EVOH, polyamide (e.g., nylon), and the like, a layer comprising regrind material, and several adhesive or tie layers to bind the other layers. See, for instance, U.S. Application No. 2004/0071904.
A large amount of trim material is typically generated when producing barrier fuel tanks by blow molding or thermoforming methods. In order to make such processes more economical the trim is reused in the process in the regrind layer. However, as the regrind layer has poor impact strength, the amount of regrind that can be used in the final structure is limited.
In current commercial operations, only about 35 wt. % regrind (based on the weight of the entire automotive fuel tank) can be incorporated into a new tank if the tank is to possess certain desirable characteristics, such as passing the cold drop (−40° C.) test. The trim in current blow molding or thermoforming operations can run as high as 60 wt. % of the weight of the fuel tank, and thus there is the problem of an excess of trim, which must then be sold as scrap.
The poor mechanical strength of regrind relative to the polyethylene layer has been addressed in the prior art by various methods, e.g., rearranging the order of layers (see, for instance, paragraph [0148] of U.S. Patent Application 2002/0176955A1) or providing a new adhesive material (see, for instance, U.S. Patent Application 2003/0175538). As multilayer structures comprise more and more diverse materials (see, for instance, EP 1108586, teaching an automotive fuel tank having a second barrier layer of amorphous carbon), the problem of the disposition of trim and other scrap material grows. It would be beneficial if a higher amount of trim could be used in the process that generates the trim, without detracting from the important properties of the final product.
U.S. Pat. No. 6,223,945 discloses a bottle resistant to stress crack comprising a multilayer resin structure having an inner layer including a metallocene polyethylene, an optional middle layer comprising post-consumer recycled resin (PCR), such as HDPE bottle scrap or LLDPE recycled pallet stretch film, and an outer layer comprising a higher density material.
U.S. Patent Application No. 2002/0086174 teaches multilayer film structures having improved seal and tear properties having a layer comprising regrind and a polyethylene having a density of from about 0.93 g/cc to about 0.97 g/cc.
WO 03/000790 A1 teaches the use of VLDPE or LLDPE polyethylene as impact modifiers for polypropylene.
EP 0622183 describes a multilayer film having a layer comprising recycled material and a primer comprising an anhydride modified polyolefin or copolymer.
Additional related references include U.S. Pat. Nos. 6,068,933; and 6,670,007; U.S. Application Nos. 2002/0051891, and 2003/0198768, Canadian Patent Application No. 2,122,283, and WO 98029245 A2.
The present inventors have surprisingly discovered that addition of low density metallocene polyethylenes to regrind allows an increase in the nount of regrind that can be used in a useful article, such as a multilayer structure comprising a regrind layer.